1. Field of the Invention
The present invention relates to a thin film transistor and a method of manufacturing the same, and, more particularly, to a thin film transistor manufactured according to a method that provides high electrified mobility, high reliability and simplified manufacturing.
2. Description of Related Art
A poly silicon layer is generally used as a semiconductor layer of a thin film transistor (TFT). The poly silicon layer is formed such that an amorphous silicon layer is first deposited on a substrate and crystallized at a predetermined temperature. A method of crystallizing the amorphous silicon layer includes an eximer laser annealing (ELA) technique, a solid phase crystallization (SPC) technique, and a metal induced lateral crystallization (MILC) technique.
Of these techniques, the MILC technique is disclosed in U.S. Pat. No. 6,097,037 and has an advantage in that the amorphous silicon layer is crystallized at a relatively low temperature and at a relatively short processing time in comparison with the ELA technique and the SPC technique.
FIGS. 1A to 1B are cross-sectional views illustrating a process of manufacturing the TFT using the MILC technique according to conventional methods.
Referring to FIG. 1A, an amorphous silicon layer 11 is formed such that an amorphous silicon is deposited on an insulating substrate 10 using a low pressure chemical vapor deposition (LPCVD) technique and patterned in the form of an island.
A gate insulating layer 12 and a gate electrode 13 are sequentially formed on the amorphous silicon layer 11 while exposing both end portions of the amorphous silicon layer 11. A high-density impurity is ion-implanted into the exposed end portions of the amorphous silicon layer 11 to form source and drain regions 11S and 11D. A non-doped portion of the amorphous silicon layer 11 acts as a channel area 11C.
A photoresist pattern 15 is formed on the amorphous silicon layer 11 and covers the gate insulating layer 12 and the gate electrode 13. At this juncture, both end portions of the amorphous silicon layer 11 are not covered with the photoresist pattern 15. Thereafter, a metal layer 14 is deposited over the entire surface of the insulating substrate 10 using a sputtering technique. Preferably, the metal layer 14 is made of Ni, Pd, Ti, Ag, Au, Al, or Sb.
Referring now to FIG. 1B, the photoresist pattern 15 is removed using a lift-off technique, whereupon offset regions 17 are formed. Subsequently, the amorphous silicon layer 11 is crystallized by a furnace to form a poly silicon layer 11a. At this juncture, a portion of the amorphous silicon layer 11 that directly contacts the metal layer 14 is crystallized by a metal induced crystallization (MIC) technique, and the offset regions 17 and the channel area 11C are crystallized by the MILC technique.
In the conventional method of manufacturing the TFT using the MILC technique, traps are prevented since the boundaries between the MIC and MILC regions are located outside the channel area 11C, for example, within the source and drain regions 11S and 11D.
However, the conventional method of manufacturing the TFT using the MILC technique additionally requires a mask process to form the offset regions 17, thereby lowering productivity and increasing the production costs.
Also, since a crystallization is performed using the MILC technique after the gate insulating layer 12 and the gate electrode 13 are formed on the amorphous silicon layer 11, an interface characteristic between the gate insulating layer 12 and the channel area 11C deteriorates, and many trap sites are provided, whereby the electric field mobility is lowered.
In addition, an MILC front 11F, including a metal silicide, exists in the channel area 11C and serves as a defect of the TFT, thereby deteriorating reliability of the TFT. Here, the MILC front 11F is referred to as that portion where lateral growths meet each other when the amorphous silicon layer 11 is crystallized by the MILC technique. Such an MILC front 11F contains more metal components than other portions and becomes a defect of the semiconductor layer.
In order to locate the MILC front 11F outside the channel area 11C, a method is introduced such that the MIC region is non-symmetrically formed centering on the channel area 11C to perform a crystallization. That method is disclosed in IEEE Electron Device Letters, vol. 21, no. 7, Jul. 2000, and is entitled “The Effects of Electrical Stress and Temperature on the Properties of Polycrystalline Silicon Thin Film Transistor Fabricated by Metal Induced Lateral Crystallization.” However, this method has a problem in that a crystallization is non-symmetrically performed and thus a processing time for crystallization is increased.